Method of modifying a finned tube for boiling enhancement

ABSTRACT

Tubing having circumferentially extending fins, either annular or helical, is modified by forming or bending the fins transversely so that the tip of each fin convolution is closely adjacent to a portion of the next adjacent fin convolution providing small continuous or discrete gaps of predetermined and controlled average size into substantially confined spaces between adjacent fin convolutions.

United States Patent 1191 Zatell Oct. 30, 1973 METHOD OF MODIFYING AFINNED TUBE 3,454,081 7/1969 Kun et al. 165/133 FOR BOILING ENHANCEMENT3,696,861 10/1972 Webb 3,696,863 10/1972 Sung Chul Kim l65/l8l X [75]Inventor: Vincent A. Zatell, Lathrup Village,

Mich FOREIGN PATENTS OR APPLICATIONS 1,533,025 7 1968 F 165 181 1 1Assrgnw Products, Des 595,680 12/1947 61127811131" 165/184 22 Fil June 11971 Primary Examiner-Charles W. Lanham Assistant Examiner-D. C. Reiley,III [21] PP N04 154,312 Att0rneyCharles R. McKinley et al.

52 us. c1. 72/68, 29/1573 AH, 29/1573 B, [571 ABSTRACT 72/367, 165/133Tubing having circumferentially extending fins, either [51] Int. Cl.B21b 15/00 nn l r r h li l, i m ifi y f rming r nding [58] Field ofSearch 29/1573 A, 157.3 AH, h fin r n v r ely so ha he ip f each finc0nv0- 29/1573 B; 165/133, 181, 184; 72/68, 367 lution is closelyadjacent to a portion of the next adjacent fin convolution providingsmall continuous or dis- [56] References Cited crete gaps ofpredetermined and controlled average UNITED STATES PATENTS size intosubstantially confined spaces between adja- 3,602,027 3 1971 Klug .1 291573 AH cent fin wnvOlunonS' 3,326,283 6/1967 Ware 165/184 X 16 Claims,12 Drawing Figures Y//,. 695 0/5 0035' 005 64 '0 O0OI,,4I a 0,'0,

METHOD OF MODIFYING A FINNED TUBE FOR BOILING ENHANCEMENT BRIEF SUMMARYOF THE DISCLOSURE It hasbeen found that heat transfer tubes adapted totransfer heat from a fluid flowing within the tube to a liquid incontact with the outer surface of the tube for the purpose of boilingthe liquid, have a substantially increased efficiency in boiling theliquid, when they are provided with a porous surface having amultiplicity of substantially confined spaces into which the liquid mayflow. These spaces have substantially restricted ports or passagesproviding for flow of liquid into the spaces and for escape of vaporfrom the substantially confined spaces.

The present disclosure relates to tubes in which heat transfer fins ofusual shape are provided on the outer surface thereof. These fins may beseparate annular fins or they may be provided to extend helically on theouter surface of the tube. In the latter case the fin convolutions maybe in the form of a single helix or two or three or more separate butinterleaved helical fins may be provided.

Preferably, the fins are formed on the tube by well known processes suchfor example as disclosed in Locke U.S. Pat. No. 1,865,575. In finnedtubing of this type the fins are integral with the material of the tubewall and are formed to extend outwardly from the tube wall by rollingoperations. As initially produced, each fin convolution extendssubstantially radially outwardly from the tube, whether provided in theform of single annular convolutions or a multiplicity of helicalconvolutions. The fins are outwardly tapered and have a height whichsubstantially exceeds the average fin thickness as well as the averagespacing between adjacent fin convolutions.

In accordance with the present disclosure, these fins are bent or formedso that the crest of some or all of the fins is closely adjacent to thesurface of the next adjacent fin convolution with the result that thereis provided a confined space of substantial size having a port orpassage leading outwardly therefrom defined between the tip of one finconvolution and the side of the next adjacent convolution.

This bending over of the finsmay be accomplished in different ways, someof which are applicable to singlestart fins and others of which aredesigned for use with .different multiple-start fins. In the simplestform of the invention the finned tubing is simply drawn through a diewhich is dimensioned to bend the fin convolutions over into requiredposition. Best results have been obtained when the dimensions andconfiguration of the die is such that the crest of each fin convolutionis not only bent over into contact with a surface of the next adjacentfin convolution, but is forced beyond the position of initial contact.When the die dimensions are properly controlled the bent over fins uponemergence from the die, spring back out of contact with the adjacentsurface and remain at a predetermined substantially accuratelycontrolled spacing therefrom. In other forms of the invention the finsare reformed by rolls or dies during or subsequent to the finningoperation.

It will be appreciated that where the fins are in the form of annularconvolutions, the confined space is annular in shape. Where theconvolutions are in the form of a single-start fin extending helicallyfrom the tube, the confined space is in the form of a single elongatedhelically disposed space. Similarly, where multiplehelix fins areprovided, separate elongated helically disposed confined spaces result.

It will further be appreciated that the closure provided by bending overthe fins does not produce a perfectly accurate continuous gapbetween teecrests of the bent-over fin convolutions, but instead, theremay be somevariation such for example as areas in which the fin tips remain incontact with the surface of the adjacent convolution and are spacedtherefrom at varying intervals. The control of the spacing mayaccordingly be considered as providing an average spacing rather than aperfectly accurate continuous spacing.

Tests of operating efficiency of the tubing modified for boilingenhancement indicate that the average width of the space or gap betweenthe crests of a fin convolution and the adjacent surface of the nextconvolution should be up to 0.007 inch. The maximum improvement inboiling efficiency is noted where the gap does not exceed 0.005 inch.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged fragmentarysection through a wall of a finned tube.

FIGS. 2-4 are sectional views showingthe configuration of fins bent overafter passage through differently dimensioned dies.

FIG. 5 is a fragmentary sectional view illustrating a somewhat differentembodiment of the disclosure.

FIGS. 6-9 are fragmentary sectional views illustrating finconfigurations for double-finned or double-start finned tubing.

FIG. 10 is a fragmentary sectional view showing one embodiment of finmodification.

FIG. 11 is a view similar. to FIG. 10 showing another embodiment of thedisclosure.

FIG. 12 is a graph showing comparative boiling efficiencies of a plaintube and a tube made in accordance with the present disclosure.

DETAILED DESCRIPTION It has been found that porous finishes, points orsmall gaps leading to partially confined spaces generally enhance theboiling capacity of a surface. Recent examples of such enhancement notedin the patent literature are tip indented finned tubing shown in US. PatNo. 3,487,670, a wire wound finned tube shown in US. Pat. No. 3,521,708,and microscopically grooved surfaces shown in US. Pat. Nos. 3,454,081and 3,496,752.

In accordance with the present invention, conventional integrally finnedtubingis modified to enhance boiling by bending the tips of the finconvolutions over so that a small gap of controlled size is formedbetween the tip of one convolution and the side or other adjacentsurface of the next fin convolution. From the foregoing it will beapparent that the small gap of controlled size leads to a substantiallyenlarged chamber or space which extends around the tube in the form ofone or more convolutions depending upon whether the fins are initiallyannular or helical.

In the first place, it has been found that by methods which willsubsequently be described, predictable small gaps up to 0.007 inch maybe produced by a simple drawing operation of the finned tube through adie having a circular cross-section and dimensioned to bend the finconvolutions as required.

Further investigation has indicated that substantial improvement inboiling efficiency is obtained when the gaps as above described, have anaverage width up to 0.007 inch with the maximum improvement where gapwidths are 0.005 inch or less.

While the simplest method of bending the fins to produce the requiredspacing is to draw the finned tube through the die, other methods are ofcourse available and may be necessary when the procedure is applied tomultiple-start fins in which different formation is imparted to theseparate fins.

Referring now to FIG. 1, there is shown in enlargement a fragmentarysection through the wall of a finned tube provided with a multiplicityof fins 14 which may, for example, be separate and independent annularfins, or provided in helical arrangement with one or more separatehelical fins. The fin of the actual tube illustrated in FIG. 1 was asingle helix; however, tubes shown in FIGS. 2, 3 and 4 could also bemade from multiple-helix fin tube.

In FIG. 2 there is illustrated the resulting deformation of the finafter the tube (which has an initial outside diameter of 0.740 inch) wasdrawn through a die of circular cross-section having an internaldiameter of 0.695 inch. The average space as indicated at 16, between'the crests of the fin convolution and the adjacent side surface of thenext adjacent fin convolution was found to be 0.0035 0.0050 inch. Aninspection of this Figure indicates that there is thus provided acontinuous substantially closed space or chamber 17 through which liquidin contact with the finned surface may enter. It will of course furtherbe apparent that where the fins 14 are in the form of separate annularfins, these enclosed spaces or chambers 17 are annular and are equal innumber to the number of fins. Where the fin is provided in the form of asingle helical fin, it will be apparent that the configurationillustrated in FIG. 2 results in a single helically extending enclosedspace or chamber. Similarly, where two or three separate but interleavedhelical fins are provided, the enclosed spaces or chambers are helicaland are equal in number to the number of separate fins.

In FIG. 3 there is illustrated the fin configuration when the samefinned tube shown in FIG. 1 was drawn through a die having an internaldiameter of 0.688 inch. In this case the gap between the crests of thefin convolutions and the adjacent side surface of the next adjacentconvolution was found to have an average width of 0.0020 0.0045 inch. Inthis Figure the location of the gap or gaps is indicated at 18. The tubeformation illustrated results in the essentially enclosed space orchamber 19 which may be circular or helical as described in connectionwith FIG. 2.

In FIG. 4 there is illustrated the fin configuration when the same tubeis drawn through a die having an internal diameter of 0.675 inch. Inthis case it was found that the crests of the fin convolution were bentover and deformed the material of the adjacent convolution as indicatedat 21, so that no gap was visible to the unaided eye. However, areproducible finite gap is visible when microscopically examined.

The three configurations of tube described in the foregoing are for thepurpose of indicating that an accurate control of the average gapbetween te crests of the tube convolutions and the adjacent side wall ofthe adjacent convolution can be predetermined and maintained byselecting the appropriate die through which the finned tube is drawn.

Tubing having the configuration illustrated in FIGS. 2, 3 and 4 has beensubjected to boiling tests and it has been found that for substantialenhancement of boiling, the gap into the enclosed annular or helicalspace should have an average width of 0.007 inch or less with themaximum improvement occurring where the gap is 0.005 inch or less.

The variable parameters which influence the efficiency of the disclosedconstruction in enhancing boiling are number of fins per inch of tubingand gap to trapped volume ratio, as well as gap width. In accordancewith the present invention, the number of fins per F l1WillP9L ljl9-Tubing generally of the type illustrated in FIGS. 2, 3 and 4 wassubjected to boiling tests in which boiling A T (which for this purposemay be considered as the difference between the average temperature ofthe tube wall metal and the boiling temperature of the fluid outside thetube) was determined for different heat flux values based on outsidearea BTU/Hr-Ft.

When the tubing was tested at 4,000 BTU/Hr-Ft, the test results were astabulated:

fins/inch Average Gap A T l 19 0.001 0.0016" 53 F. 2 19 0.0035 0.0040"53 F. 3 19 (standard fins) l l.5 F. 4 26 less than 0.001" 3.0 F. 5 260.002 0.0045" 3.0 F. 6 26 (standard fins) 120 F.

It is thus seen that the l9-fin tubing having the fins bent over todefine restricted substantially enclosed spaces represents animprovement over the unmodified finned tube in the required A T of about54 percent.

Similarly, the modified 26-fin tubing showed an improvement of aboutpercent in the boiling A T over unmodified tubing.

The foregoing tests also establish the superiority of the 26-finmodified tube over the l9-fin modified tube, in a reduction from 5.3 F.for the l9-fin tube to 3.0 F. for the 26-fin tube.

FIG. 12 graphically illustrates the enhancement in boiling which followsbending or forming the fins of a 26-fin tube, such as seen in FIG. 1, tothe form shown in FIG. 3. In this tube, where the gap between the crestsof the fins and the side surface of the adjacent fin convolution has anaverage dimension of 0.002 0.045 inch. From this graph, it will be notedthat at 6 feet per ton, the boiling AT for 26-fin standard tube is 12 F.The finned tubing modified in accordance with the present invention isreferred to as Z-fin tube, and for the 26 Z-fin tube illustrated in FIG.3, the boiling AT is 2.5 F. for an improvement of about percent in theboiling film temperature drop. 7

FIG. 5 illustrates another fin configuration for a single helix tubewhen formed by a process other than drawing through a circular die. Inthis Figure no attempt is made to illustrate a gap between the crestportion of a finned convolution and the side of the next adjacentconvolution, but it is to be understood that an average gap of less than0.007 inch will be provided.

Referring now to FIG. 6 there is illustrated a modified finned tube 24produced from a double or twostart fin. In this case each of the fins 26is inclined towards each other so that a gap of controlled dimensions isprovided at 28 between the crests of the fins. With this arrangement ahelical substantially enclosed space or chamber 29 is formed.

In FIG. 7 the fins30 are bent toward each other in such a way as todefine a gap 32 therebetween, this gap appearing between spaced apartfin portions which are relatively flat in cross-section. With thisconfiguration it will be apparent that the confined space 34 isessentially smaller than the space 29 provided between the fins in FIG.6.

In FIG. 8 an arrangement is illustrated in which one of the fins 36 isnot modified and the other fin 38 is bent towards the unmodified fin asillustrated so as to produce the helical substantially enclosed space orchamber 40 and the controlled restricted gap or opening 42. j

Referring now to FIG. 9 there is illustrated an arrangement generallysimilar to that shown in FIG. 6 except that the separate fins 44 and 46are curved and inclined towards each other to produce a substantiallycircular cross-section enclosed space or chamber 48 and the narrowrestricted gap 50.

The tubing of FIGS. and 11 is produced from finned tubing have threeinterleaved helical fins. In FIG. 10 the fins 62 and 64 are illustratedas bent toward the intermediate unmodified fin 63 so as to define thesubstantially enclosed spaces or chambers 66.

In FIG. 11 the fins, here designated 62a and 64a, are bent in adifferent manner toward the intermediate fin 63a to define thesubstantially enclosed spaces or chambers 66a.

It will be understood that in the embodiments of the inventionillustrated in FIGS. 6 to 11, the modification of the normally outwardlyor radially extending fins is provided progressively along the helicalpaths occupied by the fins. For this reason the simple embodiment of theinvention illustrated in FIGS. 2-5 is preferred, since the fins aregiven the required configuration by the simple act of drawing the finnedtube through the appropriate circular die.

While the present disclosure is broadly applicable to enhancement ofboiling in any liquid, it is particularly advantageous in the art ofrefrigeration, where the boiling or vaporization is of a refrigerant.

The finned tube as illustrated in FIG. 1, prior to bending over thefins, has in a typical example the following dimensions: The internaldiameter of the tube is 0.557 inch and the wall thickness, measured fromthe inside of the tube to the bottom of the space between adjacent finsis 0.035 inch. The fins have a radial dimension of 0.057 inch and areprovided at a frequency of about 26 fins per inch, giving a pitch fromfin convolution to fin convolution of approximately 0.038 inch. Theindividual fins have a thickness at the base of approximately 0.0165inch and at the tip or crest of 0.0075 inch. This gives an average widthof approximately 0.012 inch. The space between adjacent fins increasesfrom approximately 0.021 inch adjacent the roots of the fins toapproximately 0.0305 inch adjacent the crests.

The finned tubing illustrated in FIG. 1 has an approximate outsidediameter, before the bending of the fins, of 0.740 inch. Theconfiguration illustrated in FIG. 2

results from drawing this tube through a die having an internal diameterof 0.695 inch and produces a gap 16 into the substantially enclosedspace 17 of 0.0035 0.0050inch.

Theconfiguration illustrated in FIG. 3'results from drawing the tubingillustrated in FIG. 1 through a die having an internal diameter of 0.688inch, which produced a gap 18 communicating with the space 19 of 0.0020.004s inch.

The configuration illustrated in FIG. 4 resulted from drawing the tubingof FIG. 1 through a die having an internal diameter of 0.675 inch. Inthis operation the crests of each fin convolution were bent over intocon tact with the next adjacent fin convolution with such force as toprovide deformation of the material, as has been previously described.

The dimensions of the fins in relationship to spacing are of course ofcritical nature since this determines the general shape and dimensionsof space which is enclosed when the outer portion of each finconvolution is bent over into close proximity to the adjacent wallportion of the adjacent fin convolution, to provide the restrictedopening into the substantially enclosed space. For uniform and readilycontrollable conditions, the fin height should be greater than thespacing between adjacent fins and the fin tip thickness should besubstantially less than the fin thickness at the root. With thisconfiguration the fins are readily bent and can be formed to touch theadjacent fin, and at the same time enclose a substantial volumetricspace with accurate control of the gap providing for ingress of liquidand egress of vapor from the substantially enclosed space.

Bending of the fins is facilitated where the height of the fin issubstantially greater than the average width, as for example, not lessthan twice the average width.

In all cases the fins are formed by rolling up material from the outerwall of the tubing, so that the fins originally extend generallyperpendicular to the surface from which they were displaced, andportions thereof extend generally parallel to corresponding portions ofadjacent fin convolutions.

Since the fins are originally produced by a rolling operation, theaverage spacing between adjacent fin portions is greater than theaverage thickness of the fins. This results inherently in the productionof substantially enclosed spaces or chambers, after the outer portionsof the fins are bent over, which are of quite substantial size. At thesame time, the openings into the substantially enclosed spaces may bevery restricted, by bending the outer portions of tube convolutions intovery close spacing from the side surface of adjacent fin portions.

What I claim as my invention is:

1. The method of making tubing modified for the enhancement of boilingof liquid in contact with the exterior surface thereof which comprisesrolling up fin convolutions out of the material at the exterior of thetubing with each convolution extending generally radially outwardly ofthe tubing parallel to and spaced substantially from adjacentconvolutions, and thereafter drawing the finned tube through a diehaving a throat with a minimum transverse dimension smaller than theoutside diameter of the original fin convolutions to form the outerportions of fin convolutions completely around the finned tube intoproximity to a side of adjacent fin convolutions to define therewith asubstantially enclosed space extending around the tubing and arestricted opening into such space.

2. The method as defined in claim 1 which comprises using a diedimensioned to deform the outer portion of each fin convolution intosolid contact with the surface of the adjacent convolution, and toprovide predetermined width of the restricted opening into the space byspring-back of the outer fin convolution portion.

3. The method as defined in claim 2 which comprises using a diedimensioned to cause the outer portion of each fin convolution to deformthe portion of the next adjacent fin contacted thereby as the tubingpasses through the die.

4. The method as defined in claim 1 which comprises the initial step offorming the fin convolutions to extend helically around the tubing.

5. The method as defined in claim 1 in which the height of each finprior to bending is at least twice the average thickness thereof.

6. The method as defined in claim 1 in which the average spacing betweenadjacent fin convolutions substantially exceeds the average thickness ofeach fin.

7. The method as defined in claim 5 in which the average spacing betweenadjacent fin convolutions substantially exceeds the average thickness ofeach fin.

8. The method as defined in claim 1 in which the throat of the die isdimensioned to form the outer portions of the fin convolutions into suchproximity to the side of adjacent fins as to produce a gap having anaverage width of not more than 0.007 inch.

9. The method as defined in claim 8 in which the average width of thegap is not more than 0.005 inch.

10. The method as defined in claim 8 in which the average width of thegap is between 0.0035 0.0050 inch.

1 1. The method as defined in claim 5 in which the average width of gapis not more than 0.007 inch.

12. The method as defined in claim 5 in which the average width of gapis not more than 0.005 inch.

13. The method as defined in claim 5 in which the average width of gapis between 0.0035 0.0050 inch.

14. The method as defined in claim 6 in which the average width of gapis not more than 0.007 inch.

15. The method as defined in claim 6 in which the average width of gapis not more than 0.005 inch.

16. The method as defined in claim 6 in which the average width of gapis between 0.0035 0.0050 inch.

1. The method of making tubing modified for the enhancement of boilingof liquid in contact with the exterior surface thereof which comprisesrolling up fin convolutions out of the material at the exterior of thetubing with each convolution extending generally radially outwardly ofthe tubing parallel to and spaced substantially from adjacentconvolutions, and thereafter drawing the finned tube through a diehaving a throat with a minimum transverse dimension smaller than theoutside diameter of the original fin convolutions to form the outerportions of fin convolutions completely around the finned tube intoproximity to a side of adjacent fin convolutions to define therewith asubstantially enclosed space extending around the tubing and arestricted opening into such space.
 2. The method as defined in claim 1which comprises using a die dimensioned to deform the outer portion ofeach fin convolution into solid contact with the surface of the adjacentconvolution, and to provide predetermined width of the restrictedopening into the space by spring-back of the outer fin convolutionportion.
 3. The method as defined in claim 2 which comprises using a diedimensioned to cause the outer portion of each fin convolution to deformthe portion of the next adjacent fin contacted thereby as the tubingpasses through the die.
 4. The method as defined in claim 1 whichcomprises the initial step of forming the fin convolutions to extendhelically around the tubing.
 5. The method as defined in claim 1 inwhich the height of each fin prior to bending is at least twice theaverage thickness thereof.
 6. The method as defined in claim 1 in whichthe average spacing between adjacent fin convolutions substantiallyexceeds the average thickness of each fin.
 7. The method as defined inclaim 5 in which the average spacing between adjacent fin convolutionssubstantially exceeds the average thickness of each fin.
 8. The methodas defined in claim 1 in which the throat of the die is dimensioned toform the outer portions of the fin convolutions into such proximity tothe side of adjacent fins as to produce a gap having an average width ofnot more than 0.007 inch.
 9. The method as defined in claim 8 in whichthe average width of the gap is not more than 0.005 inch.
 10. The methodas defined in claim 8 in which the average width of the gap is between0.0035 - 0.0050 inch.
 11. The method as defined in claim 5 in which theaverage width of gap is not more than 0.007 inch.
 12. The method asdefined in claim 5 in which the average width of gap is not more than0.005 inch.
 13. The method as defined in claim 5 in which the averagewidth of gap is between 0.0035 - 0.0050 inch.
 14. The method as definedin claim 6 in which the average width of gap is not more than 0.007inch.
 15. The method as defined in claim 6 in which the average width ofgap is not more than 0.005 inch.
 16. The method as defined in claim 6 inwhich the average width of gap is between 0.0035 - 0.0050 inch.